Implement MutationObserver in the DOM polyfill

[airhorns]

What changed

@remote-dom/polyfill currently installs and documents a MutationObserver constructor, but the constructor is an empty stub: new MutationObserver(callback).observe(node) throws because observe is undefined. This is especially easy to miss when remote code is type-checked against lib.dom.

This implements the observer surface over the polyfilled tree:

• asynchronous, batched child-list, attribute, and character-data records
• subtree, attributeFilter, and old-value options
• observe(), disconnect(), and takeRecords()
• standard callback and observation-option validation

Mutation delivery is integrated alongside the existing remote synchronization hooks, so observation does not change what is sent to a Remote DOM host.

Inactive-path performance

Mutation types maintain separate active-registration flags. Mutation sites check the corresponding flag before capturing old values or allocating a record, Map, or NodeList, and before walking ancestors. The registration WeakMap is created lazily and released again when the final observer disconnects. A child-list or character-data observer therefore does not add bookkeeping to attribute writes.

In a local Node 20.20 benchmark of one million repeated setAttribute() calls with no active observers (seven measured runs after warm-up), the median changed from 66.9ms to 47.0ms compared with the initial implementation in this PR, about 30% faster.

Validation

• pnpm exec vitest run (180 tests)
• pnpm lint
• pnpm type-check
• mise exec node@20.20.0 -- pnpm --filter @remote-dom/polyfill build

[andrewiggins]

Now that #652 has landed, I'd like to implement this as a built-in Window extension consumers can opt into

[andrewiggins]

Gonna go ahead and merge this and then build on top of it